Hypothesis: In Ocs and lymphoid and myeloid cells, Cbl proteins (c-Cbl and Cbl-b) have emerged as prominent components of signaling cascades downstream of a variety of surface receptors (lymphokines, cytokines, immunoglobulin receptors and integrins). Cbl proteins mediate the assembling of signaling complexes downstream of receptor and non-receptor tyrosine kinases (RTKs and NRTKs) and, by directly interacting with them, also initiate their ubiquitination and subsequent down-regulation. The embryonic death of c-Cb1/Cb1-b double-knock out mice indicates a redundancy of key functions, consistent with the high degree of identity between Cbl proteins. However, although c-Cbl and Cbl-b are highly homologous, especially in their N-terminal phosphotyrosine binding (PTB) and the RING finger domains, there are significant differences between the two proteins in the C-terminal proline-rich (PR) and acidic regions. These differences in structural motifs will result in different patterns of molecular interactions that are likely to contribute to specific functions of each of the two proteins that cannot be compensated by the other. This is emphasized by the differences in the T cell regulation seen in the c-Cbl-/- versus Cbl- b-/- mice. These features of Cbl knockout mice have therefore raised the possibility that c-Cbl and Cbl-b regulate different signaling pathways, a fact relevant to this proposal. We have shown that c-Cbl lies downstream of the vitronectin receptor and Src and forms a tri-molecular complex with Src and Pyk2 in a signaling pathway that regulates cell adhesion and motility in Ocs and other cell types. We have also shown that deletion of the c-cbl gene altered the migration of OC both in vitro and in vivo during development, although detailed histomorphometric analysis of adult bones failed to demonstrate any changes in bone volume or resorption parameters. To further characterize the role of Cbl family proteins in Ocs, we analyzed the bones of the Cbl-b-/- mice and found that Cbl-b-/- mice were osteopenic in contrast to c-Cbl-/- mcie, indicating that Cbl-b plays a role in OC function that cannot be compensated by c-Cbl. The goal of this pilot project is to acquire data that will serve as the basis for applying for a standard R01 application for NIH. The aim of this study is to test the hypothesis that Cbl-b plays a unique role in promoting OC function and to identify the functions of Cbl-b that are required for normal regulation of osteoclastic bone resorption. The hypothesis stems from the following observations: (a) the osteopenia in Cbl-b-/- mice (b) the normal expression of c-Cbl in Cbl-b-/- Ocs, which implies that c-Cbl protein cannot compensate for Cbl-b deletion in OC function and (c) the detection of a four-fold increase in cell surface expression of receptor activator of NfkappaB (RANK) in Cbl-b-/-OCLs. (c-Cbl protein promotes the down-regulation of various receptors, and the increased levels of RANK suggest that Cbl-b may play a similar role in regulating RANK activity in OC).